Create CRM_Core_Form::addField method

We discussed the possibility to add an addfield method.
Coming from https://github.com/civicrm/civicrm-core/pull/5419

The fn signature would be pretty much the same as addSelect
After resolving the $entity and $field it would call civicrm_api3($entity, 'getfields') to load the field's metadata (aside, maybe we need to add an api "getfield" action to make it easier to fetch a single field).
Depending on the html type it would call CRM_Core_Form::add with appropriate options.
Once implemented I think all calls to addSelect could be swapped for addField and probably that function could be deleted.

A first step could be to implement the function, and change one form to use this method as much as possible.